Printing is one of the most popular ways of conveying information to members of the general public. Digital printing using dot matrix printers allows rapid printing of text and graphics stored on computing devices such as personal computers. These printing methods allow rapid conversion of ideas and concepts to printed product at an economic price without time consuming and specialised production of intermediate printing plates such as lithographic plates. The development of digital printing methods has made printing an economic reality for the average person even in the home environment.
Conventional methods of dot matrix printing often involve the use of a printing head, e.g. an ink jet printing head, with a plurality of marking elements, e.g. ink jet nozzles. The marking elements transfer a marking material, e.g. ink or resin, from the printing head to a printing medium, e.g. paper or plastic. The printing may be monochrome, e.g. black, or multi-coloured, e.g. full colour printing using a CMY (cyan, magenta, yellow, black=a process black made up of a combination of C, M, Y), a CMYK (cyan, magenta, yellow, black), or a specialised colour scheme, (e.g. CMYK plus one or more additional spot or specialised colours). To print a printing medium such as paper or plastic, the marking elements are used or “fired” in a specific order while the printing medium is moved relative to the printing head. Each time a marking element is fired, marking material, e.g. ink, is transferred to the printing medium by a method depending on the printing technology used. Typically, in one form of printer, the head will be moved relative to the printing medium to produce a so-called raster line which extends in a first direction, e.g. across a page. The first direction is sometimes called the “fast scan” direction. A raster line comprises a series of dots delivered onto the printing medium by the marking elements of the printing head. The printing medium is moved, usually intermittently, in a second direction perpendicular to the first direction. The second direction is often called the slow scan direction.
The combination of printing raster lines and moving the printing medium relative to the printing head results in a series of parallel raster lines which are usually closely spaced. Seen from a distance, the human eye perceives a complete image and does not resolve the image into individual dots provided these dots are close enough together. Closely spaced dots of different colours are not distinguishable individually but give the impression of colours determined by the amount or intensity of the three colours cyan, magenta and yellow which have been applied.
In order to improve the veracity of printing, e.g. of a straight line, it is preferred if the distance between dots of the dot matrix is small, that is the printing has a high resolution. Although it cannot be said that high resolution always means good printing, it is true that a minimum resolution is necessary for high quality printing. A small dot spacing in the slow scan direction means a small distance between marker elements on the head, whereas regularly spaced dots at a small distance in the fast scan direction places constraints on the quality of the drives used to move the printing head relative to the printing medium in the fast scan direction.
In order to move over a receiver in the fast-scan direction several printheads may be located on a single shuttle, moving over the receiver, guided on a guide rail. Generally, there is a mechanism for positioning the marker elements in a proper location over the printing medium before it is fired. Usually, such a drive mechanism is controlled by a microprocessor, a programmable digital device such as a PAL, a PLA, a FPGA or similar although the skilled person will appreciate that anything controlled by software can also be controlled by dedicated hardware and that software is only one implementation strategy.
One general problem of dot matrix printing is the formation of artefacts caused by the digital nature of the image representation and the use of equally spaced dots. Certain artefacts such as Moiré patterns may be generated due to the fact that the printing attempts to portray a continuous image by a matrix or pattern of (almost) equally spaced dots. One source of artefacts can be errors in the placing of dots caused by a variety of manufacturing defects such as the location of the marker elements in the head or systematic errors in the movement of the printing head relative to the printing medium. In particular, if one marking element is misplaced or its firing direction deviates from the intended direction, the resulting printing will show a defect which can run throughout the print. A variation in drop velocity will also cause artefacts when the printing head is moving, as time of flight of the drop will vary with variation in the velocity. Similarly, a systematic error in the drive system for moving the printing medium may result in defects that may be visible. For example, slip between the drive for the printing medium and the printing medium itself will introduce errors.
Especially in large size inkjet printers and industrial inkjet printing machines, the receiving medium transport system has to be very accurate and reliable in transport distance to avoid banding problems.
In certain printers the medium is held on a platen roller, usually having a relative large diameter, but such printers are restricted in the kind of media they are capable of handling. No relatively rigid media can be handled.
Another aspect in industrial printers is that the shuttle containing the printheads is usually relatively heavy in comparison to home or office printers. Due to the higher shuttle speed in industrial printers, the drops when release by an inkjet printhead follow a sloped path from the printhead to the receiver. Even the slightest deviation in throw distance, i.e. the distance between the head and the receiver will result in deviations in positioning the ink drops. In order to avoid misplacement of dots, the throw distance has to be kept constant over the full width of the shuttle and over the full length of the shuttle movement.
Also other recording processes need a constant printhead/receiver distance, e.g. to ensure equal impact along the pringting area.
Small printers usually have a single guide rail or two guide rails positioned on the same side. In industrial printers this give rise to problems as the shuttle, due to higher weight, generates considerable torque forces upon the guide rails as the heavy shuttle will deform the usually large size guide rails giving variations in the throw distance, resulting in problems for guarding recording quality.
Another well known aspect in digital printing is that the distance between printhead and receiver has to be adjusted to compensate for media thickness, in inkjet this is needed in order to keep the throw distance constant.
In EP 336 870 the position of a printhead is adjusted automatically or manually with respect to a platen roller in accordance with the thickness of a recording medium.
In a much more recent US 2004/17 456 several elements of the print engine, e.g. receiver table, printheads, shuttle, guide rails, can be adjusted likewise to compensate for media thickness.
However, adjustments of all these elements simultaneously makes adjustment difficult and may give rise to errors in positioning the printhead.
In US 2004/17 456 it is suggested to use a dynamic compensation device using a distance sensor to keep the throw distance constant, but is a complicated, expensive and relative unreliable method as it is an active method using moving parts.
The system also uses a total of four guide rails for carrying the printhead shuttle. This diminishes the problem of torque but even so the guide rails will slightly bend lowering the throw distance in the middle of the recording table.
A problem not recognised is that due to thermal expansion the shuttling system will develop strain resulting in deformations and probably inferior printing quality.
It is clear that there is still a need for improvement of large size printing systems to keep the printhead-receiver distance constant over the recording area.